From AMB Labs Website:
β22 ("beta 22") is a high-end, reference-quality DIY stereo amplifier for dynamic headphones. It could also be built as a power amplifier for speakers (18Wrms into 8Ω unbalanced, or 50Wrms into 8Ω fully-balanced), or as a preamplifier.
The all-discrete (no integrated circuits), fully complementary and cascoded topology, high-current MOSFET...